Field
The present invention relates to methods and apparatus for probing the subsurface of the earth using electrical fields. More particularly, the invention relates to the generation and measurement of an electrical field oriented in an orthogonal direction to the axis of a cased borehole.
Background
The embodiments described herein relate generally to soundings within the Earth based upon electrical fields. As used herein, “Earth” generally refers to any region in which a borehole may be located including, for example, the lithosphere.
Electromagnetic (EM) geophysical soundings probe electrical conductivity in the ground as a function of depth. Typical targets of interest include ore bodies, hydrocarbons, water, and environmental pollutants. Since the conductivities of such targets and the surrounding medium may be quite dissimilar, they may be discriminated by means of measurement of their subsurface conductivity when subjected to an electromagnetic field. Using this methodology, the depth, thickness, and lateral extent of materials of interest may be determined.
The source of the EM field used in a geophysical sounding may originate in the natural environment, or be manmade. If manmade the source may produce a primarily magnetic field or electrical field that varies in time and this primary field produces a secondary field in the conducting earth. For example an electrical field produces electrical currents in the earth that have an associated magnetic field, and a time varying magnetic field induces electrical currents that result in an electrical field. The electrical properties of the earth and rate of change of the field determine the relative magnitudes of the secondary and primary fields. The combination of primary and secondary fields results in combined electromagnetic interaction with the earth even for a source arranged to produce solely an electrical or magnetic field.
While the majority of EM geophysical soundings are performed with sensors and EM sources on the surface of the Earth, a borehole can provide physical access to the subsurface. Measurement of the electrical or magnetic field within a borehole can be related to the electrical or magnetic field in the earth around the borehole, or the fields that would exist in the earth in the absence of the borehole. Similarly, connecting an electrical field or magnetic field source to the Earth via a borehole provides a way to produce fields within the Earth at desired depths without the attenuation and uncertainties that may result if the source fields originated from a source at the surface of the Earth.
A common factor in electrical field-based geophysical soundings is the need to couple an electrical circuit to the Earth in order to measure or apply an electrical potential. In the simplest embodiment for measurement, the local electrical potential is coupled into an amplifier by an electrical conductor, or electrode in contact with the earth. For soundings in a borehole, the simplest approach is to remotely insert an electrode in a bore in a location adjacent an area of interest, like a hydrocarbon-bearing formation. However, boreholes are typically filled with fluid which gives rise to increased electrode noise due to streaming potentials along with noise related to motion of the sensors within the borehole. As a result, electrical field based methods that require high sensitivity measurements have not been applied in a borehole.
Furthermore most boreholes are lined with metallic tubular known as casing or liner that provides good electrical conductance. For electromagnetic soundings based on magnetic fields, casing produces a small distortion in the magnetic field that is being produced or sensed. However, for EM soundings based on electrical fields, casing has a significant effect and must be taken into account when arranging an electrode that is coupled to the Earth. One approach is to locate the electrode or electrodes on the outside of the casing. However, this approach is unrealistic as the casing is typically cemented in the borehole and any electrode installed at the time the casing is inserted would have to have a service life comparable to that of the casing. Because conventional “galvanic” electrical contact between an electrode and the Earth requires electrochemical exchange of electrode atoms with the surrounding earth and fluids, the electrodes inevitably fail early on. In addition, the presence of the casing in such close proximity to the electrode causes significant distortion of the fields that the electrode is intended to measure.
There exists a need for methods and apparatus for measuring electrical fields in a borehole while avoiding problems associated with galvanic contact and that provides measurement and generation of electrical fields orthogonal from the borehole.